OPTICAL COMMUNICATION SYSTEM, AND TRANSMITTER AND RECEIVER APPARATUS THEREFOR

• US 20130177316A1
• Filed: 01/06/2012
• Published: 07/11/2013
• Est. Priority Date: 01/06/2012
• Status: Abandoned Application

• Current Assignee: Emcore Corporation
• Original Assignee: Emcore Corporation
• Inventors: Cardakli, Mustafa
• Application Number:
  US13/345,213
• Publication Number:
  US 20130177316A1
• Time in Patent Office: Days
• US Class Current: 398/79
• CPC Class Codes: H04B 10/5053 using a parallel, i.e. shunt, combination of modulators
• CPC Class Codes: H04B 10/5167 Duo-binary; Alternative mark inversion; Phase shaped binary transmission
• CPC Class Codes: H04B 10/532 Polarisation modulation
• CPC Class Codes: H04B 10/614 comprising one or more polarization beam splitters, e.g. polarization multiplexed [PolMux] X-PSK coherent receivers, polarization diversity heterodyne coherent receivers H04J14/06 takes precedence
• CPC Class Codes: H04J 14/06 Polarisation multiplex systems

Abstract

An optical communication system having a transmitter in which a pair of optical signals having different frequencies are modulated using a duobinary encoding scheme, and then multiplexed using polarization division multiplexing. Advantageously, the frequency difference between the two signals can be less than the data rate conveyed by each signal, resulting in a narrow spectral bandwidth, while still allowing demultiplexing at a receiver using simple bandpass filters and without the need of any form of polarization tracking. A receiver has a beam splitter for splitting the received optical signal into two portions which are each directed, via respective bandpass filters centred at slightly different frequencies, to respective detectors. Advantageously, the frequency difference between the frequencies at which the bandpass filters are centred can be less than the data rate of a detected signal. The receiver does not require any polarization tracking or balancing, and accordingly is straightforward to implement

Claims

1. A transmitter comprising:

a first light source for generating a first coherent light beam at a first frequency;

a first encoder for encoding a first data signal using a duobinary encoding scheme to generate a first encoded signal;

a first modulator for modulating the first coherent light beam in accordance with the first encoded signal to generate a first modulated light beam, wherein the first modulated light beam has a first polarization state;

a second light source for generating a second coherent light beam at a second frequency different from the first frequency;

a second encoder for encoding a second data signal using a duobinary encoding scheme to generate a second encoded signal;

a second modulator for modulating the second coherent light beam in accordance with the first second encoded signal to generate a second modulated light beam, wherein the second modulated light beam has a second polarization state that is different from the first polarization state; and

a combiner arranged to combine the first modulated light beam and the second modulated light beam for form a combined light beam for output to a communication channel.

2. A transmitter according to claim 1, wherein the first and second data signals have a data rate which is greater then a frequency difference between the first frequency and the second frequency.

3. A transmitter according to claim 1, wherein the first encoder and the second encoder comprises:

a precoder for differentially encoding the first data signal to generate a differential signal;

a drive circuit for generating a drive signal in dependence on the differential signal; and

a low pass filter for filtering the drive signal to generate a duobinary drive signal.

4. A transmitter according to claim 3, wherein the precoder comprises:

an inverter for inverting the first data signal to generate an inverted data signal; and

a logic gate for performing an exclusive-OR logic function on the inverted data signal and a feedback data signal to generate an output signal, the feedback data signal corresponding to a time-delayed version of the output signal.

5. A transmitter according to claim 3, wherein the low pass filter is a fifth order Bessel filter.

6. A transmitter according to claim 1, wherein one or both of the first modulator and the second modulator comprises a Mach Zehnder modulator.

7. A transmitter according to claim 1, wherein the first polarization state and the second polarization state are linear polarization states which are orthogonal to each other.

8. A transmitter according to claim 1, wherein the combiner is a polarization beam combiner.

9. A transmitter according to claim 1, wherein the first frequency and the second frequency are 20 GHz or more.

10. A transmitter according to claim 9, wherein a frequency difference between the first frequency and the second frequency is less than 20 GHz.

11. A receiver comprising:

a wavelength-dependent beam splitter arrangement operable to split a received optical signal, which is modulated in accordance with a data signal and has a first spectral component centred at a first frequency and a second spectral component centred at a second frequency different from the first frequency, into a first optical signal and a second optical signal, wherein the first optical signal comprises more of the first spectral component than the second spectral component and the second optical signal comprises more of the second spectral component than the first spectral component;

a first detector for detecting the first optical signal; and

a second detector for detecting the second optical signal,

wherein the first detector and the second detector are operable to detect a data signal having a data rate that is greater than the frequency difference between the first frequency and the second frequency.

12. A receiver according to claim 11, wherein the wavelength-dependent beam splitter arrangement comprises:

a beam splitter operable to split the received optical signal into a first split optical signal and a second split optical signal;

a first bandpass filter operable to filter the first split optical signal to form the first optical signal, the first bandpass filter being centred at the first frequency; and

a second bandpass filter operable to filter the second split optical signal to form the second optical signal, the second bandpass filter being centred at the second frequency.

13. A receiver according to claim 12, wherein the first bandpass filter and the second bandpass filter have a 3 dB bandwidth corresponding to the frequency difference between the first frequency and the second frequency.

14. A receiver according to claim 11, wherein the wavelength-dependent beam splitter arrangement comprises an optical de-interleaver.

15. A receiver according to claim 11, wherein the wavelength-dependent beam splitter arrangement comprises a wavelength demultiplexer.

16. A receiver according to claim 11, wherein the first detector and the second detector are operable to detect a data signal having a data rate of 20 GHz or more.

17. A receiver according to claim 11, wherein the frequency difference between the first frequency and the second frequency is less than 20 GHz.

18. An optical communication system comprising:

a plurality of transmitters for generating modulated optical signals, each modulated optical signal being centred at a respective different optical frequency;

a multiplexer for multiplexing said modulated optical signals for simultaneous transmission over an optical communication link;

a demultiplexer for demultiplexing said modulated optical signals following transmission over the optical communication link; and

a plurality of receivers for detecting the modulated optical signals,

wherein each transmitter is arranged to generate a pair of duobinary modulated optical signals centred at respective optical frequencies separated by less than the data rate of each of the pair of duobinary modulated optical signals.